DNA Folding in Sperm

In sperm, DNA is tightly compacted to create a small, hydrodynamic sperm head. This dramatic reorganization of the nucleus is carried out by protamine proteins that fold the DNA into loops and toroids. Here, I'll talk about the pathway and physical mechanism of that DNA folding.  We will look at images of single molecules of DNA with bound protamine using an atomic force microscope (AFM) and also real-time folding dynamics using an optical trapping assay. We expected DNA folding in sperm to be similar to DNA folding in bacteria with DNA looping happening in a single step. We also thought that forming a DNA toroid would follow the “one ring” model where a single ring or loop of DNA acts as the nucleation event for the toroid. But, our data showed that DNA folding in sperm is completely different than our expectations. We are now using our findings to fold and assemble DNA origami nanostructures.